The invention concerns a receiver for radio-frequency signals transmitted by transmitter sources, in particular of the GPS type. The receiver has means for improving the reception dynamic of said signals for example when the signals are masked by an obstacle. Said receiver includes receiving and shaping means with frequency conversion for the radio-frequency signals for generating intermediate signals, a correlation stage formed of several correlation channels for receiving the intermediate signals in order to correlate them, in operating channel control loops, with carrier frequency and specific code replicas of visible transmitting sources to be searched and tracked, each channel being provided with a correlator in which at least one integrator counter is capable of providing, at the end of each determined integration period of the correlated signals, a binary output word whose value compared to a determined detection threshold level allows to detect the presence or absence of the visible transmitting source to be searched and tracked, and microprocessor means connected to the correlation stage for processing the data extracted, after correlation, from the radio-frequency signals. In the case of a GPS receiver, the data extracted from the signals are in particular the GPS message and pseudo-ranges.
The radio-frequency signal receiver of the present invention can of course also be used in a satellite navigation system of the GLONASS or GALILEO type. Likewise, the receiver could be used in a mobile telephone network, for example of the CDMA type (Code-division multiple access). In such case, the transmitting sources are no longer satellites but base cells of the telephone network, and the processed data concerns audible or legible messages.
In the current GPS navigation system, 24 satellites are placed in orbit at a distance close to 20,200 km above the surface of the Earth on 6 orbital planes each offset by 55° with respect to the equator. The time taken by a satellite to make a complete rotation in orbit before returning to the same point above the Earth is approximately 12 hours. The distribution of the satellites in orbit allows a terrestrial GPS receiver to receive GPS signals from at least four visible satellites to determine its position, velocity and the local time for example.
In civil applications, each of the satellites in orbit transmits radio-frequency signals formed of a carrier frequency L1 at 1.57542 GHz on which are modulated a pseudo-random PRN code at 1.023 MHz peculiar to each satellite and a GPS message at 50 Hz. The GPS message contains the ephemerides and almanac data from the transmitting satellite, which are useful in particular for calculating the X, Y, Z position, velocity and time.
The PRN code (pseudo random noise), in particular of the Gold code type, is different for each satellite. This Gold code is a digital signal formed of 1023 chips which are repeated every millisecond. This repetition period is also defined by the term Gold code “epoch”. It is to be noted that a chip takes the values 1 or 0 as for a bit. However, a chip in the GPS technology is to be differentiated from a bit which is used to define a unit of data.
The Gold codes defined for 32 satellite identification numbers have the characteristic of being orthogonal. By correlating them with each other the correlation result gives a value close to 0. This characteristic thus enables several radio frequency signals transmitted on a same frequency originating from several satellites simultaneously to be independently processed in several channels of the same GPS receiver.
Currently, in several daily activities, GPS receivers which are portable or incorporated particularly in vehicles are used to allow navigation data to be provided to users. This data facilitates orientation towards the desired target and allows users to have the knowledge of their bearings. Moreover, portable GPS receivers are of smaller size so as to enable them also to be incorporated in objects which can easily be transported by one person, such as in cellular telephones or in wristwatches. However, in these objects of small dimensions, it is often necessary to minimise the energy consumed by the receiver, as they are powered by a battery or accumulator of small size.
A GPS receiver needs to pick up the radio-frequency signals transmitted by at least four visible satellites in order to determine in particular its position and time related data. However, the receiver can pick up the almanac data peculiar to each satellite by locking on individually to one of the visible satellites.